As a preparation of exposing system API. This patch does some
harmless refactoring, which includes:
1. Change raw arrays into lists according to API guidelines.
2. Write test.
3. Rename class to UnderlyingNetworkInfo.
4. Rename vpnIface to iface.
5. Make underlyingIfaces @NonNull in order to adapt new
unparceling code.
6. implement equals and hashCode for testing.
Test: atest android.net.UnderlyingNetworkInfoTest
Bug: 174123988
Change-Id: I405c21e57c4af8a12a9dd0a1749b9e6690f87045
2050bed66e